Mechanosynthesis of the Fast Fluoride Ion Conductor Ba1–xLaxF2+x: From the Fluorite to the Tysonite Structure

Ba 1– x La x F 2+ x covering the whole range of compositions was synthesized by high-energy ball milling of mixtures of BaF 2 and LaF 3 at ambient temperature. The compounds obtained in this way crystallize in the cubic fluorite structure in the range from 0 ≤ x ≲ 0.775 which extends the range for the fluorite-type Ba 1– x La x F 2+ x covered up to now (0 ≤ x ≤ 0.55) considerably. By employing 19 F NMR spectroscopy and X-ray (total) scattering, indications for a continuous change from the fluorite (BaF 2 ) to the tysonite structure (LaF 3 ) were found. The mechanosynthesized samples showed slightly higher fluoride ion conductivities for the samples with x ≤ 0.40 and clearly smaller conductivities in the case of the samples with x ≥ 0.85 than reported for their monocrystalline counterparts. Two conductivity maxima at x ≈ 0.40 and x ≈ 0.85 and a conductivity minimum at x ≈ 0.75 were observed for the mechanosynthesized Ba 1– x La x F 2+ x . For the samples with 0.10 ≤ x ≤ 0.50, exhibiting a dc conductivity in the order of 10 –5 S/cm at 400 K, all fluoride ions seem to be highly mobile at temperatures beyond 453 K. The decrease of the ionic conductivity observed for Ba 1– x La x F 2+ x with increasing x in the range from x ≈ 0.50 to x ≈ 0.75 is accompanied by a change of the diffusion behavior and a decrease of the ratio of highly mobile to nonmobile or slow fluoride ions.